Chapter 15 Unified Threat Management for FortiOS 5.0 : ICAP : Configuration Settings : Servers : Maximum Connections
  
Maximum Connections
This value refers to the maximum number of concurrent connections that can be made to the ICAP server. The default setting is 100. This setting can only be configured in the CLI.
The syntax is:
config icap server
edit <icap_server_name>
set max-connections <integer>
end